Metal layers in integrate circuits are utilized to electrically interconnect various devices fabricated on a substrate. Resistance of the materials utilized in the metal layers affect the speed with which signals can propagate between these devices. To improve the propagation of signals, copper has taken over as the primary metal in use in high speed design applications.
Nevertheless, even with increase conductivity of copper, vis-á-vis aluminum, speed issues with copper interconnect exist. For example, as copper conductor features continue to decrease in size, the conductivity resistance associated with the copper conductors increases causing a decrease in speed of signals on these size decreased signal traces. Another problem that affects the propagation of signals is an increase in the electromigration resistance. As conductor feature sizes continue to decrease copper electromigration resistance is limited by such things as surface diffusion and voids.